1. Field of the Invention
The present invention concerns a combination of compounds having perfluorinated alkyl chains suitable for use for the surface treatments of substrates.
2. Description of the Related Art
Compounds containing perfluorinated alkyl chains are well known to have low intermolecular forces and have been used effectively in surface treatment of materials to provide stain resistance, hydrophobicity, and oleophobicity especially where the surface energy of the treated surface should be less than about 30 mN/m.
Such perfluorinated alkyl chains are usually attached to functional groups which can bind to a surface to which they are applied, or else they may be attached to a polymerizable moiety to form a monomer which can be polymerized to form a polymer having perfluorinated side chains which can then be formulated into a coating for application to the surface to be treated. Alternatively, coatings containing such monomers may be applied to the surface and polymerized in situ.
The surface properties of fluorinated polymers such as poly(meth)acrylates depends on the pendant perfluoroalkyl groups. Polymers with short fluorinated chains are not able to form an ordered surface at ambient temperatures because of high thermal molecular mobility. Fluorinated polymers with longer perfluoroalkyl groups do form an ordered surface under ambient conditions and the resulting increased concentration of trifluoromethyl groups in the top layer provides high water and oil repellency. For these reasons fluorinated polymers with long perfluorinated side chains have been widely used for surface coatings.
U.S. Pat. No. 5,641,844 relates to polymers of acrylic or methacrylic acid esters that have pendant fluoroalkyl side groups which provide durable water and oil repellency to substrates coated with these polymers. The fluoroalkyl groups having the formula —(CH2)m(CF2)nF with n=8, 10, 12, 14, etc enhance repellent performance thanks to the aligned fluoroalkyl side chains which are locked in a quasi-crystalline configuration, limiting their thermal mobility. Thus long, pendent chains of fluorinated carbons are required to significantly improve the repellency of materials coated with polymers of acrylic or methacrylic acid esters.
WO 96/05172 relates to acrylic monomers which contain urethane units and perfluorinated alkyl groups. The reference shows that, to provide optimum water and oil repellency, the pendant perfluorinated alkyl groups should be long chained and should be permanently aligned parallel to adjacent pendant groups attached to adjacent acrylic backbone units. In these polymers, the pendant perfluoroalkyl groups are modified by the presence of urethane groups between the acrylic backbone and the perfluorinated groups. The affinity of the urethane groups for one another apparently helps to keep the entire pendant chain substantially in a fixed alignment even when subjected to adverse conditions, such as heat. This aspect is generally referred to as a matrix effect, which produces alignment resulting in the apparent growth of the polymeric chain.
US 2008/0146734 provides improved oil-resistant and self-cleaning surfaces by coating with copolymers comprising common monomers such as acrylic acid, hydroxyethyl methacrylate and methyl methacrylate with fluorinated monomers having a long, fluorinated alkyl group linked to the acrylate group via a hydrophilic group.
However, due to concerns about the environmental persistence and bioaccumulative nature of compounds containing perfluorinated alkyl chains of seven or more carbon atoms, such as perfluorooctanoic acid (PFOA), regulations restricting the use of materials which can decompose or be metabolized to such molecules are becoming more strict. Thus, significant research is being undertaken into alternative chemistries, which rely on compounds containing shorter perfluorinated chains having six or fewer fluorine-bearing carbon atoms.
Compounds with a reduced perfluorinated chain length (fewer than seven fluorine-bearing carbon atoms) can provide a low surface energy if used in bulk materials or in relatively thick coatings (several μm thick). However when used in thin coatings, especially under one micrometer thickness, their performance in reducing surface tension is poor compared to monomers having perfluorinated chains of seven or more carbon atoms. This is especially the case in the presence of oils or greases. While a high degree of water repellency can be achieved using short chain perfluorinated molecules, oil repellency of such materials is poor in comparison with those based on longer chain perfluorinated molecules. Honda et al, in Macromolecules, 2005, 38, 13, 5699 teach that ordered structures form easily between parallel perfluoroalkyl chains of greater than eight carbons, while such pseudo-crystalline structures form to a much lesser extent between perfluoroalkyl chains having fewer than eight carbon atoms, and such pseudo-crystalline structures which do form between shorter perfluoroalkyl chains are easily disrupted. This could be interpreted as due to weaker hydrophobic interactions (Van der Waals forces) between shorter chains which are, therefore, unable to maintain an organized layer with well aligned perfluoroalkyl chains.
Oil and water repellency obtainable using coatings based on short-chain perfluoroalkyl molecules may be improved by engineering the properties such as the glass transition temperature and the molecular weight of copolymers of fluorinated and non-fluorinated acrylic monomers as described in, for example, U.S. Pat. No. 8,193,277. However, such copolymers are not usable in all circumstances.
The addition of polar functionalities, for example groups able to form hydrogen bonds, between a cross-linkable group and a short perfluoroalkyl chain has been observed to give improved performance in thin layers (Clegg and Hutchison et al, J. Am. Chem. Soc. 1999, 121, 5319-5327), possibly due to association between polar molecular segments forcing a stronger aggregation and order in the short, perfluoroalkyl chains. However, this still may not reach the high level of performance achievable using long-chain (seven or more carbon atoms) perfluoroalkyl derivatives.
Thus, it is desirable to develop more environmentally acceptable fluorinated compositions having perfluorinated alkyl chain lengths of six carbons or fewer in order to meet future environmental protection standards which, when formulated into coatings or surface treatments, provide hydrophobic and oleophobic surface properties equivalent to those produced hitherto using coatings containing longer perfuorinated alkyl chains.